Thermally reversible networks were reported by Frye, J. Org. Chem., 34 (9), 2496-9 (1969) for the base catalyzed ring-opening polymerization of spirocyclotetraalkoxysilanes. The thermal reversibility was attributed to the relatively high stability of the spirosiloxane rings, that is, the polymerization exhibited a positive enthalpy and a positive entropy. The network was not a rubber since the cross-linking density was too high. In other spiroalkoxysilane systems, thermally reversibility was not observed. See, e.g., Kipping et al., J. Chem. Soc. 81-84 (1944). The spriosiloxanes are generally difficult to prepare in high yield.
U.S. Pat. No. 2,793,222 (Kantor et al.) describes the production of organopolysiloxane elastomers by incorporating into a polymerizable organopolysiloxane mixture the cross-linker 1,2-bis-heptamethylcyclotetrasiloxanylethane ("ditetramer"), which is a dimer of octamethylcyclotetrasiloxane. The acid catalysts described by Kantor et al., ferric chloride and sulfuric acid, are too weak to result in formation of a living rubber.
Bis-heptamethylcyclotetrasiloxane-yl-methane and bis-heptamethylcyclotetrasiloxane-yl-ethane were prepared by Gilbert and Kantor, J. Polym. Sci. 40, 35-38 (1959), by the action of peroxides on octamethylcyclotetrailoxane. The resulting yield was poor and the only cross-linking functionality which may result is tetrafunctional. Networks could be prepared, but thermal reversibility was not reported. The copolymerization of ditetramer in the presence of a base catalyst, tetra-n-butylphosphonium hydride is described. A base catalyst will not result in formation of a living rubber.
Mayo et al., J. Polymer Sci. 55, 65 (1961) discloses a tetrafunctional cross-linker which, under the appropriate conditions, could result in living rubber formation. However, the reference recites polymerization in the presence of a base catalyst. A living rubber therefore did not result.
An improved synthesis for certain polycyclosiloxanes via hydrosilation chemistry was later reported. For example, the trifunctional polycyclosiloxane1,3-di-(2-heptamethylcyclosiloxane-yl-ethyl)-1,1,3,3-tetr amethyldisiloxane was prepared in high yield and purity from vinylheptamethylcyclotetrasiloxane and tetramethyldisiloxane (Andrianov et al., Izv. Akad. Nauk SSSR. Ser. Kim., 1975, 9, 2055-8). The polymerization of this compound and its use as a cross-linker in a rubber composition are unreported.
U.S. Pat. No. 4,826,710 (Buese) describes bicyclosiloxane imides which may be cured in the presence of an acid catalyst to a hard network, or optionally cured with a cyclic siloxane to effect the formation of a flexible network. Living behavior was not reported since the networks were too highly cross-linked.